Slackermedia
The Workflow Edition
Qtractor
A pro Digital Audio Workstation for audio and MIDI composition with an extensive plug-in architecture providing as many soft synths and effects as you can fit into one fermata.
Audacity
The famous all-purpose audio workhorse. Enough said.
JACK
Take audio routing to the next level. String together audio, MIDI, samples, synths, effects, applications, and speakers, and then save the session for later.
LMMS
An all-in-one music composition and mixing application with enough loops, presets, and effects to keep you busy all the way from dc to al coda.
MuseScore
Write your composition in a WYSIWYG editor, listen to MIDI playback of your music, and then print sheet music and a conductor's score.
and much more.